The present invention relates to a method and apparatus for treating groundwater or other water streams contaminated with oxygenates.
In response to the 1990 Clean Air Act Amendments gasoline suppliers began to blend fuels with oxygenates, such as alkyl ethers, particularly methyl-t-butyl ether (MTBE). Unleaded gasoline often comprises as much as 10 to 15% by volume of MTBE. Alcohol oxygenates such as ethanol and t-butyl alcohol (TBA) have also been used for the same purpose.
After using oxygenated fuels for over a decade, it has become apparent that these cleaner burning fuels pose distinct threats to groundwater resources. In particular, many oxygenates are very soluble in water and are slow to degrade in the environment; hence they tend to accumulate in water resources including surface waters and groundwater once released into the environment.
Due to leaks in underground storage tanks or spills, the groundwater at many gasoline retail, distribution, and manufacturing sites is contaminated with benzene, toluene, ethyl benzene, and xylene (BTEX), as well as MTBE and other ethers. For example, MTBE has been detected in groundwater with high frequency in many sites and there are well documented cases of impacts to municipal water supply wells. Due to the fact that MTBE and other ethers are characterized by the properties of high solubility in water, relatively low volatility compared to BTEX, relatively low carbon sorption coefficient, and poor biodegradability, the ethers are more easily transported in groundwater aquifers than BTEX and do not degrade through natural attenuation. In addition oxygenate alcohols such as TBA are also found as contaminants in groundwater since they are often present as impurities in the oxygenate ether feedstocks and/or are breakdown products of the ethers.
The presence of oxygenate contaminants in groundwater and in particular public water supplies poses serious problems since the ether oxygenates have very low odour and taste threshold concentrations. Typically regulators and local authorities require that potable water supplies should not contain more than 20 xcexcg/L (ppb), and in some cases as low as 5 xcexcg/L (ppb) of MTBE.
Granular activated carbon (hereafter GAC) has been used for treatment of wastewater and contaminated groundwater at the surface. In xe2x80x9cA Review of Potential Technologies for the Treatment of Methyl tertiary Butyl Ether (MTBE) in Drinking Waterxe2x80x9d, discussing a study by Anthony Brown et al., University of Southern California Department of Civil and Environmental Engineering of the Metropolitan Water District of Southern California, City of Santa Monica, the authors mention the use of GAC, along with polymeric resins and chemically modified clays, but state at page 136 that adsorbability is low on GAC, adsorption capacity for MTBE is low, and frequent GAC regeneration is required. (API-National Ground Water Association xe2x80x9cPetroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection and Remediation Conference, Houston Nov. 12-14, 1997).
Thus, while MTBE can be removed from recovered groundwater by treatment with granular activated carbon beds (GAC), it is relatively expensive compared to the treatment of BTEX because the GAC beds are subject to frequent exhaustion. Equally important, GAC is not effective at all on tertiary butyl alcohol (TBA) that is found along with MTBE in contaminated groundwater and is the primary metabolite in the biodegradation of MTBE, and is equally poorly biodegradable.
Where groundwater contaminated with BTEX, MTBE, and other ethers is treated using activated carbon there is a need in the art for a method which reduces the need for frequent changing of the carbon bed and which also addresses the problem of degrading alcohol oxygenates such as tertiary butyl alcohol (TBA).
The use of bacteria or naturally occurring microbes for biodegradation of a wide range of organic contaminants is known. However, attempts to degrade MTBE and/or TBA using bacterial cultures have generally met with little success.
For example, K. Mo, et al. Appl. Microbiol. Biotechnol. (1997) 47:69-72 proposes isolating from activated sludge and fruit of the gingko tree three pure cultures, belonging to the genera Methylobacterium, Rhodococcus, and Arthrobacter, that are capable of degrading MTBE. However, the data presented by Mo proposes that only a minor portion of the MTBE was degraded by the cultures and very little if any, of MTBE degraded to carbon dioxide within the time frame of the experiment.
An exception to the difficulties encountered in degrading MTBE and/or TBA with bacterial cultures is described in U.S. Pat. Nos. 5,750,364 and 5,902,734, which disclose mixed bacterial cultures capable of biodegrading MTBE and TBA to carbon dioxide and water, and U.S. Pat. No. 5,811,010, which describes aerobic degradation of t-butyl alcohol using activated sludge.
A sample of a mixed bacterial culture prepared according to U.S. Pat. No. 5,750,364 has been deposited with the American Type Culture Collection (ATCC), Patent Depository, 12301 Parklawn Drive, Rockville, Md. 20852, USA, with ATCC number 202057, under the Budapest Treaty (see also Column 2, line 64 to Column 3, line 4 of U.S. Pat. No. 5,902,734). Samples of this culture can be obtained from the permanent collection of the ATCC, Patent Depository (and Column 3, lines 1 to 4 of U.S. Pat. No. 5,902,734 indicate that all restrictions imposed by the depositor on the availability to the public were to be irrevocably removed upon granting of U.S. Pat. No. 5,750,364 (issued May 12, 1998) or of U.S. Pat. No. 5,902,734 itself.
WO 00/63343 describes a pure bacterial culture isolable from mixed bacterial culture ATCC No. 202057, and capable of degrading methyl t-butyl ether (MTBE) to carbon dioxide.
It is apparent from the art that it is more difficult to degrade MTBE and other ethers than BTEX due to the properties of the ethers. The ethers have high solubility in water, relatively low volatility compared to BTEX, relatively low carbon sorption coefficient, poor biodegradability, and are more easily transported in groundwater aquifers than BTEX. MTBE can be removed from recovered groundwater by physical adsorption with a GAC bed, but due to the fact it is not very hydrophobic and the capacity for sorption is not as high as BTEX, it is relatively expensive to remove MTBE by this method compared to BTEX due to frequent exhaustion of the activated carbon beds. In addition, activated carbon is not effective at all on TBA which is often found along with MTBE contaminated groundwater, and is even less hydrophobic.
The use of immobilized biological reactors, in which a biomass of bacteria and/or other microorganisms is retained on the surface of activated carbon particles is a known option for the treatment of contaminated waters. For example, xe2x80x9cExperiences with GAC-Fluid Bed for Bioremediation of BTEX-Contaminated Groundwatersxe2x80x9d, G. Mazewski, J. Tiffany and Hansen, Biotechnol. Ind. Waste Treat. Biorem., (Pub. 1996)333-344(1992) describes a demonstration project and a full scale remediation project wherein groundwater from an operating recovery well at a bulk storage terminal was treated using a bioreactor of fluidised granular activated carbon particles. In this work the removal of BTEX was more satisfactory than the removal of other compounds such as MTBE.
Further, xe2x80x9cBioreactor Treatment of MTBE and TCE In Contaminated Ground Waterxe2x80x9d, by Miller, Michael E., et al, from In Situ and On-Site Bioremediation, Pap. Int. In Situ On-Site Biorem. Symp., 4th (1997), Vol. 5, 89-94, describes a study at the Sparks Solvent/Fuel Site (Sparks, Nev.) wherein ground water containing MTBE, BTEX and various chlorinated solvents was treated in two granular activated carbon-fluidised bed bioreactors operating in parallel. For the first few weeks after reactor startup, 85% of the influent MTBE was removed, however effluent MTBE concentrations soon increased, and MTBE removal efficiencies dropped to 10-15% indicating that the initial removal was predominately due to sorption. Later carbon containing unidentified MTBE-degrading cultures was added to one of the fluidised bed bioreactors and the MTBE removal efficiency in that reactor increased to about 75%.
Fluidised bed reactors, as employed in the above references, comprise a bed of activated carbon particles completely fluidised by a uniform upward flow of liquid at a velocity sufficient to ensure movement of individual particles throughout the fluidised bed. As fluidised beds are well mixed they are not susceptible to blockages on account of solids in the feed stream or growth and build-up of biomass on the carbon particles. However, the use of fluidised bed activated carbon bioreactors is not appropriate for achieving the high removal efficiencies required for oxygenates such as MTBE and TBA from groundwater destined for public water supply. For example, a MTBE removal efficiency of greater than 90%, and usually closer to 99%, is more often required to meet the treated water concentrations of 5-20 xcexcg/L (ppb) specified by many authorities.
For the high efficiency removal of contaminants such as MTBE it would be advantageous to use a packed bed reactor (also known as a fixed bed reactor). Packed bed reactors are distinguished from fluidised bed reactors in that they comprise a closely packed bed of activated carbon in the reactor, and liquid may be passed through the bed in any direction. The bed may be restrained from movement so that high flow through of liquid may be used without promoting fluidisation or mixing of the activated carbon particles. Packed beds of granular activated carbon are often preferred to fluidised beds as they are more efficient in terms of volume, cost of operation as well as removal of contaminants.
However, attempts by the Applicant to inoculate packed beds of granular activated carbon with bacteria by conventional means of introducing bacteria into the bed together with a contaminated water feed stream have proven problematic, as the bacteria are only distributed in the entry region of the bed and this leads to loss of performance due to plugging of the bed, flow channelling and bypassing.
There is therefore a need in the art for a method of treating groundwater contaminated with more recalcitrant chemicals such as MTBE and TBA. In addition, where an activated carbon bed is used to assist in the removal and degradation of MTBE, there is a need for a method that reduces the need for frequent replacement of the carbon beds. Furthermore, there is a need for a method that also provides for the degradation of TBA.
The present invention provides a method and apparatus for degrading oxygenates, including, but not limited to, ethers, alkyl ethers and alkyl alcohols, particularly branched alkyl ethers/alcohols, more particularly tertiary carbon atom-containing alkyl ethers/alcohols, and still more particularly MTBE and TBA, which reduces the need for the frequent replacement of activated carbon beds and, at the same time, allows for the removal of TBA where it would otherwise have not occurred. Accordingly, the invention provides a method of treating groundwater or other water stream(s) contaminated with an oxygenate to degrade said oxygenate which comprises:
a) inoculating a biodegrader capable, of degrading said oxygenate on an activated carbon bed, preferably a packed activated carbon bed, such as a packed activated carbon bed, by means of a rigid tubular instrument having a plurality of holes in the part of the rigid tubular instrument used for inoculation of the carbon bed; and
b) flowing said groundwater, or other water stream contaminated with said oxygenates through a structure having a top, bottom, and sides, and a predetermined volume containing said bed of activated carbon having said biodegrader inoculated thereon.
In this regard the present invention provides a method for the purification of groundwater contaminated with oxygenates such as alkyl ethers and tertiary butyl alcohol, and to a method and apparatus that result in the efficient biodegradation of these compounds to carbon dioxide and water. In particular, the invention provides for the remediation of groundwater contaminated with methyl-t-butyl ether (MTBE) and other ethers and alcohols using granular activated carbon (GAC), such as packed (fixed) beds of granular activated carbon (GAC), seeded with specific MTBE degrading bacteria cultures, and to a new method of effectively inoculating the carbon beds with the cultures which avoids plugging whilst allowing even distribution of the bacterial culture throughout the bed. By means of the new method of inoculation, the activated carbon may be, in a preferred embodiment of the present invention, temporarily xe2x80x9cfluidisedxe2x80x9d in the region of inoculation allowing better distribution of the culture and potential adherence of the culture to the carbon.